LIKE most famous Paris monuments, Pamela Digby Churchill Hayward Harriman, the American Ambassador to France, has been built on a series of foundations dating back to earlier times: first there was Pamela Digby, the upper-class English party girl and gold digger; then there was Pamela Churchill, the globe-trotting mistress of rich men; then Pamela Hayward, the showbiz socialite; and finally Pamela Harriman, the finished, fully restored edifice: wealthy widow, raiser of funds (and hackles) on behalf of the Democratic Party and latterly queen of the American Embassy in Paris -- a rare distinction for a woman once considered persona non grata at the British Embassy, a few doors away, on account of her racy reputation.

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It is a breathtaking structure, this life, making up in sheer grandeur and scale what it lacks in good taste, and in Sally Bedell Smith it has found a worthy archeologist. Where Christopher Ogden in his 1994 biography ''Life of the Party'' approached the subject with a hatchet, and a rather blunt one at that, Ms. Smith comes armed with tweezers, toothbrush and a microscope. Over five years and 400 interviews, the author has left no stone unturned and no layers of slung mud unexcavated to reveal Mrs. Harriman the promiscuous social mountaineer and naked self-promoter alongside Mrs. Harriman the powerful Democratic doyenne and woman of substance.

Ms. Smith occasionally veers from biography into vivisection as she slices away at the myth. Mrs. Harriman, who has got away with so much over the years, gets away with nothing here. Her chins are double, her sense of irony quite lacking and her intellect second-rate, we are told. Her French is not quite up to scratch and even the hat she wears for her first wedding is ''unbecoming.'' Some of the knife work is not for the squeamish, and readers may wish to skip the detailed description of the techniques employed by Mrs. Harriman's plastic surgeon, who ''would remove the subcutaneous fat around the eyes and below the jawline, cut off the excess skin, pull and 'redrape' the facial skin back and upward,'' and so on.

Yet at the end of Ms. Smith's operation, the literary equivalent of a reverse face lift, Mrs. Harriman somehow emerges scarred but whole and, if anything, still more formidable with warts and all. The biographer's dislike for her subject boils on the page, but she is too honest to disguise the grudging admiration that goes with it. The Pamela Harriman she paints is both monstrous and magnificent. Even her reflected glory is dazzling.

''What is your secret?'' Barbra Streisand once asked Mrs. Harriman at a White House correspondents' dinner. The answer, according to Ms. Smith, is men: men as partners, financiers or political patrons, and most crucially as mirrors.

The infant Pamela, it seems, was already in little doubt about what the opposite sex was for. ''Man will come and man will fix,'' she is said to have remarked when her mother's car broke down. Seven decades later, her son Winston (now well into middle age but forever ''young,'' poor fellow) observes: ''I don't think she is ever truly happy without a man in her life.''

In the interim Mrs. Harriman collected three husbands and an array of millionaires, aristocrats, diplomats, captains of industry, playboys, politicians and, we are invited to assume, many others. Men came and men fixed, each adding a little to Mrs. Harriman's reflected luster and, in many instances, her bank balance. It was an astonishing achievement for a debutante who arrived in prewar London blessed with not much more than a long aristocratic pedigree, a good hunting seat and a cursory education.

Her first husband was said to be physically repellent, vulgar, pompous, lecherous, loud, hopelessly indebted and usually awash with drink. He was, on the other hand, Randolph Churchill, son of Britain's great wartime leader, and a long-term investment in power nomenclature that Mrs. Harriman proudly carries to this day. Randolph was making love to another woman when his own son was born; before that, he had read Gibbon out loud to his wife in bed, so he probably deserved everything he got, but one of Mrs. Harriman's quoted rationales for marrying him is chilling. ''I was getting so terribly upset by seeing all my friends going off, as they dramatically thought, to be killed, and I thought how marvelous it was to be going out with somebody about whom I didn't give a damn.''

From her inevitably failed marriage to Randolph, we follow Mrs. Harriman through the beds and lifestyles. Edward R. Murrow, Gianni Agnelli, Aly Khan, Stavros Niarchos and Elie de Rothschild, to name only the first division. Most of these relationships appear emotionally arid. In the ''well-navigated'' Mrs. Harriman, the men got a pliant and socially skilled mistress and in return she got glamour, power, cachet and cash. No one seemed to give much of a damn about anyone or anything else, least of all the Sixth Commandment.

During her World War II affair with Averell Harriman, the diplomat and millionaire railroad heir, the betrayals are shared equally: while they are both committing adultery in London, their spouses are doing the same in New York and Cairo. Even her later marriages, first to the Broadway producer Leland Hayward and then to Harriman himself, are portrayed as arrangements of convenience more than affection.

We are presented with an image of a courtesan so adept that none would wed her. ''Nobody marries Pam Churchill,'' Leland Hayward's wife objected when he announced he was leaving her for the flame-haired temptress.

Ms. Smith is at her best in charting Mrs. Harriman's interesting approach to the facts as she constructed her personal mythology. ''My own firsthand knowledge of France goes back to 1936-1938 when I studied at the Sorbonne,'' Mrs. Harriman told the Senate committee approving her appointment to Paris. The author shoots back immediately: ''Without the slightest embarrassment, several months grew to two years, with the implication that she had received a prestigious degree.'' A thick-skinned chameleon, Mrs. Harriman appears infinitely adaptable: while with Gianni Agnelli she developed a bogus Italian accent; for her job in Paris she swotted up on trade agreements. The technique was the same, and highly effective. Such chutzpah enraged her contemporaries (particularly those with husbands), but she just went plowing on, stroking egos, making contacts, heading upward.

''It was always 'I'm moving on to the next,' never 'he's tired of me,' '' her friend Lydia Redmond recalled of Mrs. Harriman's astonishing amatory progress. Few British exports claim such durability.

Ms. Smith records an ''absence of joy'' in this odyssey, yet one gets quite the contrary impression. For most of her life Mrs. Harriman was clearly having a ball, albeit with a packed and complicated dance card. After the war, Mrs. Harriman once recalled, ''I woke up one day and said 'My God, you've never had any fun.' '' Bear in mind that she had spent most of the war years going to parties, having affairs and reveling in the company and patronage of Sir Winston Churchill, the most amusing and powerful man in Britain, and you have a woman with a most demanding definition of ''fun.''

Mrs. Harriman's early frivolity, as distinct from her later incarnations as Democratic den mother and foreign affairs expert, was surely a key ingredient in her allure. She could be witty if she cared to -- she turned to a neighbor at one of Elie de Rothschild's dinner parties and suggested he try some of the Chateau Mouton Rothschild ''because it's homemade, you know'' -- but she is permitted a rare joke in 560 pages.

In other ways the portrait is finely balanced, and the fullest we are likely to get. Mrs. Harriman emerges as hard, dismissive and snobbish, a poor mother and a worse stepmother, but also as a woman with an innate knack for survival who turned social charm into an art form and who, perhaps as a result, has proved to be a most effective power broker and diplomat. Pamela Harriman changed herself, her aims and her partners with bewildering speed and completeness, but several character traits were made of more durable stuff, including a set of brass knuckles and an iron will, that will long outlast her.